You might have seen many gold ornaments with intricate designs, sometimes embedded with precious stones. Gold is definitely one of the most precious metals on the earth. But, is the gold that is being used to craft jewellery 100% pure?
The answer is no. The gold we use in ornaments is not a pure metal but is a mixture of several other metals.
But, why can't we use pure gold instead? Pure gold is extremely soft which is an advantage to making wonderful designs as it moulds into any shape, but is also a disadvantage as it can be easily deformed. To bring stability, firmness and strength to gold, it is mixed with copper, nickel and zinc, thus producing an 'alloy' of metals. Alloys are thus stronger than individual metals.
In a 22karat gold ornament, around 91.6% is gold while the remaining 8.4%(approx) is usually copper, zinc and nickel. Each of these metals has a specific purpose that adds up to the unique properties of an alloy.
Bronze, an alloy of copper and tin is the first alloy to be developed. This led to the evolution of mankind from Stone Age to Bronze Age where many weapons and tools were made out of bronze. Many fascinating inventions happened in the Bronze Age because of this remarkable invention.
Fig. 1: Gold bullion bars | Public domain | Wikimedia Commons
Let us now look into alloys, how they are made, types and examples.
- We will first explain to you what an alloy is and how they are prepared.
- Then we will go over the types of alloys classified based on the arrangement of atoms.
- We will then look into a few examples of alloys
- We will conclude by describing the properties of alloys.
Alloy Definition
An alloy is a mixture of two or more substances whose primary component is a metal.
Just like metals, most alloys have metallic bonding.
Alloys are prepared by melting the metals, then mixing them and allowing them to cool, and solidify.
An alloy doesn't necessarily contain only metals. It can also be formed in combination with non-metals like carbon, but a general rule of thumb is at least one of the combining substances must be a metal.
Alloys are stronger and harder as compared to pure metals. Pure metals have a crystal structure just like ionic solids, but the particles in pure metals are not locked in their positions, unlike ionic solids. Cations of metals are arranged in layers among a sea of electrons. These layers slide over one another. Hence, when external pressure is applied to pure metals, they cannot withstand the pressure and bend because of the movement of particles. This property is called the malleability of metals.
Fig. 2: Structure of metal with cations(in red) in sea of electrons | Created using images from Wikimedia Commons
To offer extra strength and hardness to metals, they are alloyed with other substances. Doing so will result in the distortion of the metal's crystal structure locking the particles in place. The particles will sit in their places when external pressure is applied, making them a rigid solid. Hence, the properties of alloys are superior as compared to pure metals.
If two are more metals are chemically combined in definite proportions, then such a substance is called an intermetallic compound. Alloys have metals and other elements mixed in random proportions.[1] So, alloys must not be confused with intermetallic compounds.
Types of Alloys
Based on how the atoms are arranged in an alloy, they are classified into two types:
- Substitutional alloy.
- Interstitial alloy.
Substitutional Alloy
Imagine a box of red apples. Pick a few apples randomly and replace them with green apples. The result should look something like this which is similar to the atomic arrangement that you might find in a substitutional alloy.
Fig. 3: Pure metal vs substitutional alloy | Created using the images from wikimedia commons (Shared under public domain)
Substitutional alloys form when the mixed elements in the alloy have similar atomic radii. The atoms of one element can substitute into the crystal lattice structure of the other element.[2]
A substitutional alloy is formed when the size of the atoms of the metal that are being replaced is roughly similar to that of the size of the atoms of the element that is replacing them.
Bronze, an alloy of 88% copper and 12% tin, is an example of a substitutional alloy. Both copper and tin have similar-sized atoms in them. The primary metal is copper whose atoms are replaced by tin atoms.
Interstitial Alloy
Again imagine a box of red apples. Even if they are tightly packed, small spaces will be left between them. If you drop in a handful of blueberries, they occupy the spaces between the apples.
Can you observe the difference between the substitutional and interstitial alloys? The element with a smaller atomic radius will not have to replace the atoms of primary metal. Instead, they just occupy the spaces between the atoms of the primary metals-Interstitial spaces.
Interstitial alloys form when the mixed elements in the alloy have different atomic radii, and the smaller atoms fill in the interstitial spaces (holes) in the lattice structure of larger atoms.[2]
Fig. 4: |nterstitial alloy | Created using images from Wikimedia Commons shared under public domain
Steel is an example of interstitial alloy. Steel contains iron and some amount of carbon. The carbon atoms are smaller than the iron atoms hence slip into the interstices of the iron matrix.
Now that we have understood the types of alloys, let us move on to some examples of alloys.
Examples of Alloys
Given below are a list of some common alloys and their applications.
Alloys | Components | Applications |
Alnico | Iron + aluminium, nickel, cobalt, plus other metals such as copper and titanium | Magnets in loudspeakers and pickups in electric guitars. |
Amalgam | Mercury + silver, tin, copper, and zinc | Dental fillings |
Brass | Copper + zinc | Door locks and bolts, brass musical instruments, central heating pipes |
Bronze | Copper + tin + manganese, phosphorus, aluminium, or silicon | Decorative statues, musical instruments. |
Cast iron | Iron + carbon + silicon | Metal structures such as bridges and heavy-duty cookware |
Cupro nickel | Varying proportions of copper and nickel. | Currency coins |
Duralumin | Aluminium + copper + magnesium + manganese | Automobile and aircraft body parts, military equipment. |
Gun metal | Copper , tin ,zinc and phosphorus | Guns, decorative items |
Invar | Iron + nickel | Pendulum clocks and scientific instruments that need to resist heat expansion. |
Nichrome | Nickel + chromium | Firework + ignition devices. Heating elements in electrical appliances. |
Steel | Iron + chromium + small amount of carbon, nickel, manganese, molybdenum, and other metals | Jewellery, medical tools and cookware. |
White gold | Gold palladium, silver, copper. | Jewellery |
An alloy of mercury is called amalgam.
Properties of Alloys
As discussed in previous sections, alloys are superior in quality compared to individual metals. Let us have a brief look into the properties of alloys:
Better strength : The alloy steel which is made up of iron and carbon is tougher than the parent element iron. Alloys are stronger and harder than the parent metals because of the disruption of the metal's crystal structure as discussed in the previous sections.
Corrosion-resistance: Another reason to employ alloys is their better performance against corrosion, which is a natural and unwanted process of decaying elements. If you take the example of the alloy bronze, which is a mixture of copper and tin, it is less likely to corrode than the parent element copper itself. Thus, alloys are resistant to corrosion, hence more durable.
Low malleability: Although malleability is a useful property of a metal, it can be disadvantageous at times. Consider gold, for instance, an extremely malleable metal so much so that if you make a ring out of its pure form, it easily gets deformed. Extreme malleability will not help retain the shapes of the materials we design. Hence, alloying reduces the malleability to some extent so that the material stays in shape
Low Melting points: Alloys have low melting points than the corresponding pure metals. Hence, moulding a metal is easier than moulding a pure metal, thus making the manufacture of goods easier and more economical.
With this, we conclude the topic-Alloys. Now, have a look around your home and school and make a note of the alloys you find. Interview your history teacher about the Bronze Age and how bronze was invented.
Alloys - Key takeaways
- An alloy is a mixture of two or more substances whose primary component is a metal.
- Alloys are of two types- Substitutional and interstitial alloys.
- Bronze, Brass and cast iron are some of the examples of alloys
- Pure metal is too soft to be useful. It is highly malleable and doesn't retain the shape and is deformed easily. This is because the atoms are arranged in layers, in pure metal, which slide over each other.
- Alloys, on the other hand, are less malleable as compared to pure metal. Hence, alloys are strong, corrosion-resistant and less malleable. This property is because the gaps are filled in, and the particles are locked in place. Hence, the layers do not slide over each other offering stability and rigidity to alloys.
References
- (2017). Alloy vs. intermetallic compounds: Effect of the ordering on the electrocatalytic activity for oxygen reduction and the stability of low temperature fuel cell catalysts. Applied Catalysis B: Environmental, 217, 201-213. https://doi.org/10.1016/j.apcatb.2017.05.081
- https://www.studysmarter.co.uk/explanations/chemistry/ionic-and-molecular-compounds/interstitial-and-substitutional-alloys/
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